Multi-channel pipetting systems include a multi-channel sampling pipette intended for the calibrated aspiration of liquids and the subsequent deposition of the liquids into receptacles. Sampling pipettes can be held in the hand of an operator during aspiration and dispensation of the liquid or may be included as part of an automated system. As an example, a multi-channel pipette may include a body forming a handle, in addition to a bottom part which, at its end, includes a plurality of pipette sampling tubes on which sampling cones can be positioned. The bottom part includes a fixed body which, at its bottom end, has a plurality of sampling tubes spaced from each other in a sideways direction relative to the pipette body, with each sampling tube including an orifice communicating with an intake chamber.
Parallel pistons are positioned in the intake chambers, and mounted at their upper ends to a piston holder that can be translated relative to the fixed body. The piston holder may be controlled by a manual or a motor-driven arrangement that causes the piston holder to rise during the liquid sampling or aspiration phase and causes the piston holder to descend during the liquid transfer phase, with the upward motion achieved, in general, by the expansion of a spring that was compressed during the downward movement.
During its movement, the piston holder carries with it the pistons that are attached to it so that the pistons are capable of simultaneous motion in a sliding movement that is usually parallel to a longitudinal central axis of the pipette. As an example, upward motion imposed on the piston holder, and therefore on the pistons, determines the volume of liquid drawn up. The volume is selected beforehand by the user, for example, using a control knob, an adjusting screw, a digital keypad, etc.
The piston holder usually includes a guide rod parallel to a longitudinal central axis of the pipette, mounted to slide on the fixed body by means of two bearings spaced along the longitudinal central axis. The piston holder additionally includes a piston support head attached to the bottom end of the rod and accommodating the top end of the pistons. In this case, the piston support head is arranged parallel to the lateral direction, and therefore orthogonal to a longitudinal central axis of the pipette.
The precision and accuracy of the volume aspirated depends on the ability to reproduce, for each piston, the same simultaneous distance of travel for a given volume setting. However, it has been observed that during the use of a multi-channel pipette, the support head of the piston holder, also known as the “rake”, has a tendency to move without maintaining orthogonality with the sliding direction resulting in a slightly inclined plane determined by the lateral direction and the sliding direction. This effect, known as the “rake effect,” occurs during the reciprocating motion of the piston holder and results in a difference in the distance of travel for one piston relative to another piston thereby reducing the precision and accuracy of the volume aspirated between the plurality of pistons. The rake effect may be caused by a large amount of play between the guide rod and the guide bearings, by an unsuitable shape of the return spring that is used to raise the piston holder, by inadequate stiffness of the piston holder, by an imbalance in the friction of the sealing gaskets mating with the pistons, etc.